Interpretive Summary: Serotonin is a primary neurotransmitter regulating aggression and impulsive behaviors. The present study investigates the effects of two different pharmaceutical agents (selective receptor antagonists) on two strains of domestic roosters. The roosters used in this study were selected for susceptibility or resistance to Marek's disease. Previous studies have shown these strains also differ greatly in their aggressiveness profiles. The present study determined that roosters from the high aggressive strain responded differently to the pharmaceutical agents than did roosters from the low aggressive strain. Our data show evidence of genetic differences in regulating aggression through the serotonin pathway. The results from the present study can be used to help identify genetic markers that can be used in breeding programs to select birds with reduced aggressiveness.

Technical Abstract:
1. Serotonin (5-HT) is a primary regulating neurotransmitter involved in aggressive and impulsive behaviors in mammals and birds. Previous studies have also demonstrated the function of serotonergic system in regulating aggression is affected by both genetic and environmental factors.
2. Our objective was to investigate the aggressive and impulsive behavioral response to antagonism of the 5-HT1A and 1B receptors in roosters bred for susceptibility and resistance to Marek’s disease (i.e., strain 72 and 63, respectively).
3. Roosters from strain 72 exhibited increased aggressive behaviors compared to strain 63 roosters, and reduced levels of brain 5-HT.
4. Antagonism of 5-HT1A receptors increased aggressiveness in strain 72, but not strain 63 roosters. 5-HT1B receptor antagonism had no effect on aggression in either strain.
5. Serotonin turnover, but not absolute central 5-HT levels, was altered in both strains following 5-HT1B antagonism, but only in strain 72 roosters following 5-HT1A antagonism.
6. Our data suggests 5-HT1A and 1B regulate aggression differently in high and low aggressive strains.